The popularity of wireless communication systems, such as cellular telephones, has lead to the need for different techniques to allow multiple users to communicate simultaneously. Some existing wireless communication systems support different users transmitting both voice and data.
A code division multiple access (CDMA) system assigns specific codes to users, thus allowing a base station to separate out individual communications based on the code. A time division multiple access (TDMA) system assigns specific time slots for communication to ensure that user communications do not interfere with each other. These systems suffer from several drawbacks including the complexity required for both the transmitting device and the base station, the inability for all users to transmit at once, and a reduced system capacity based on the number of available codes or time slots.
Some systems that attempt to solve some of these drawbacks use the Aloha protocol. One version of the Aloha system allows multiple users to transmit simultaneously but there is no spreading of the signal. This system suffers from reduced capacity for an available transmission bandwidth. Another version of the Aloha system spreads the signal throughout the available transmission bandwidth. This systems offers increased capacity but may not be suitable for real-time communication, such as voice, that depends on a guaranteed transmission rate.